Tolerances and surface finish significantly determine the price of your CNC part. Understanding IT classes and Ra values helps you specify smartly and avoid unnecessary costs.
What Are IT Tolerance Classes?
| IT Class | Tolerance at Ø50mm | Typical Use | Cost Factor |
|---|---|---|---|
| IT14 (General) | ±0.30 mm | Non-functional dimensions | 1x |
| IT7 (H7/g6) | ±0.012 mm | Sliding fit, bearings | 2x |
| IT6 | ±0.008 mm | Precision bearings | 3x |
What Do Ra Values Mean?
| Ra Value | Appearance | Typical Use | Cost Factor |
|---|---|---|---|
| Ra 3.2 | Light machining marks | Standard engineering | 1x |
| Ra 1.6 | Smooth | Sealing surfaces | 1.5x |
| Ra 0.8 | Very smooth | Hydraulics, pneumatics | 2x |
Rule of Thumb
Each tighter IT class = approximately 20-30% more cost for that dimension. Specify smart: general tolerance ISO 2768-m for most dimensions, tight tolerances only where needed.
Frequently Asked Questions
What does IT7 mean for CNC parts?
IT7 is an ISO tolerance class allowing ±0.012mm deviation at 50mm. It's used for sliding fits and bearing seats. IT7 requires finish machining and costs about twice as much as general tolerance (IT14).
What Ra value is standard for CNC machining?
The standard Ra value is Ra 3.2 µm. For sealing surfaces, Ra 1.6 is recommended; for hydraulics, Ra 0.8. Each finer step adds ~50% cost. Related: How to order CNC parts correctly.
What is ISO 2768-m?
ISO 2768-m is the medium general tolerance for CNC parts. At 50mm, it allows ±0.3mm deviation. Sufficient for most non-functional dimensions.